1. Field of the Invention
The present invention relates to a photoelectric conversion device and a semiconductor device having a photoelectric conversion element. Specifically, the present invention relates to a photoelectric conversion device formed from a thin film semiconductor element and a manufacturing method thereof. Moreover, the present invention relates to an electronic device using a photoelectric conversion device.
2. Description of the Related Art
A number of photoelectric conversion devices generally used for detecting an electromagnetic wave are known, and for example, a photoelectric conversion device having sensitivity to ultra-violet rays to infrared rays is referred to as a light sensor (also referred to as photosensor) in general. A light sensor having sensitivity to light in a visible region with a wavelength of 400 nm to 700 nm is particularly referred to as a visible light sensor, and a large number of visible light sensors are used for devices which need illuminance adjustment or on/off control depending on human living environment.
In particular, in a display device, brightness in the periphery of the display device is detected so as to adjust display luminance thereof. This is done because unnecessary electric-power can be reduced by detecting ambient brightness to obtain appropriate display luminance. For example, such a light sensor for adjusting luminance is used for a cellular phone or a personal computer.
In addition, as well as ambient brightness, luminance of a back light of a display device, particularly, a liquid crystal display device is also detected by a light sensor to adjust luminance of a display screen.
In such a light sensor, a photodiode is used for a sensing part and an output current of the photodiode is amplified in an amplifier circuit. As such an amplifier circuit, for example, a current mirror circuit is used.
FIG. 2A shows a cross-sectional view of a conventional light sensor and a manufacturing method thereof (see Reference 1: PCT International Publication No. 04/068582).
First, a metal film 1102 is formed on a first substrate 1101. A single layer or a laminate formed of an element selected from W, Ti, Ta, Mo, Cr, Nd, Fe, Ni, Co, Zr, Zn, Ru, Rh, Pd, Os, and Ir or an alloy material or a compound material containing the above element as a main component can be used. Alternatively, a single layer or a laminate of nitride thereof may be used. The thickness of the metal film 1102 is 10 nm to 200 nm, preferably 50 nm to 75 nm.
Next, an insulating film 1103 is formed on the metal film 1102. At this time, an amorphous metal oxide film 1100 is formed between the metal film 1102 and the insulating film 1103 with a thickness of about 2 nm to 5 nm. When peeling is performed in a subsequent step, the separation occurs within the metal oxide film 1100, at the interface between the metal oxide film 1100 and the insulating film 1103, or at the interface between the metal oxide film 1100 and the metal film 1102.
As the insulating film 1103, a film is formed of silicon oxide, silicon oxide containing nitrogen, or a metal oxide material by sputtering or plasma CVD. It is desirable that the thickness of the insulating film 1103 be twice the thickness of the metal film 1102 or more, preferably 150 nm to 200 nm.
Next, a film formed of a material containing at least hydrogen is formed on the insulating film 1103. As the film formed of a material containing at least hydrogen, a semiconductor film, a nitride film, or the like can be used. A semiconductor film is formed here. After that, heat treatment for diffusing the hydrogen contained in the film of a material containing at least hydrogen is performed. This heat treatment is performed at a temperature of 410° C. or higher, and may be performed separately from a forming process of a polysilicon film or may be performed as well as for the forming process of a crystalline semiconductor film to reduce the number of steps. For example, in the case where an amorphous silicon film containing hydrogen is used as the material film containing hydrogen and heated to form a polysilicon film, if heat treatment is performed at a temperature of 500° C. or higher for crystallization, diffusion of hydrogen can be performed at the same time as the polysilicon film is formed.
Then, by a known technique, the polysilicon film is etched into a desired shape to form thin film transistors (TFTs) are formed. Each TFT has a polysilicon film 1105 including a source region, a drain region, and a channel forming region, a gate insulating film which covers the polysilicon film 1105, a gate electrode formed on the channel forming region of the polysilicon film 1105, and a source electrode 1107 and a drain electrode 1108 which are connected to the source region and the drain region through an interlayer insulating film 1119. Note that the interlayer insulating film 1119 is formed of a plurality of insulating films which isolate a gate electrode from a source electrode and a drain electrode.
Next, a photoelectric conversion element connected to a source electrode 1107 of a TFT over the interlayer insulating film 1119. Here, a diode is formed as the photoelectric conversion element. First, a first electrode 1110 connected to the source electrode 1107 is formed and then an amorphous silicon film 1111 which is a photoelectric conversion layer and a second electrode 1112 are formed thereon. After that, the amorphous silicon film 1111 and the second electrode 1112 are etched into a desired shape to form the diode. After that, a wiring 1113 connected to the second electrode of the diode is formed and at the same time, a wiring 1114 connected to the drain electrode 1108 and an output terminal is formed.
Then, a second substrate 1115 to be a support for fixing a semiconductor film is attached with an adhesive material 1116. Note that it is preferable that a substrate which is more rigid than the first substrate 1101 is used as the second substrate 1115. Typically, a glass substrate, a quartz substrate, a metal substrate, a ceramic substrate, or a plastic substrate can be used as the second substrate 1115. In addition, an adhesive material made of an organic material is preferably used as the adhesive material 1116. Here, a planarization layer may be formed on a part of the adhesive material. Here, the planarization layer may be formed by applying a water-soluble resin 1116a to an adhesive material formed of an organic material and attaching a member 1116b (hereinafter referred to as a double-sided sheet), each side of which is covered with a reactive peeling adhesive material thereonto thereby attaching the TFT 1104 and the diode (including the electrode 1110, the amorphous silicon film 1111, and the electrode 1112) to the second substrate 1115. Using the attachment method, a subsequent separation process can be carried out with relatively weak force. As the adhesive material made of an organic material, a variety of peeling adhesive materials such as a reactive peeling adhesive material, a heat peeling adhesive material, a light-peeling adhesive material such as a UV peeling adhesive material, and an anaerobic peeling adhesive material can be given.
In FIG. 2B, the first substrate 1101 and the metal film 1102 formed thereover are collectively referred to as a release body 1150. Further, layers from the insulating film 1103 to the wiring 1113 connected to the second electrode of the diode and a wiring 1114 connected to an external terminal of the diode are collectively referred to as a stack body 1151.
Next, the metal film 1102 over the first substrate 1101 is peeled from the insulating film 1103 by a physical means. Physical force is relatively low force, for example, load using a member having a sharp end of a wedge shape or the like, wind pressure of gas blown from a nozzle, or ultrasonic waves. Separation occurs within the metal oxide film 1100, at the interface between the insulating film 1103 and the metal oxide film 1100, or at the interface between the metal oxide film 1100 and the metal film 1102, so that the stack body 1151 and the release body 1150 are tom off from each other with relatively low force. Thus, the stack body 1151 can be detached from the release body 1150.
Then, a third substrate 1117 and the insulating film 1103 (that is the stack body 1151) are bonded with a bonding material 1118 as shown in FIG. 2C. As the third substrate 1117, a plastic substrate or a member formed of an organic resin is used. It is preferable that a plastic substrate made of PET (polyethylene terephthalate), PEN (polyethylene naphthalate), PES (polyether sulfone), polypropylene, polypropylene sulfide, polyearbonate, polyetherimide, polyphenylene sulfide, polyphenylene oxide, polysulfone, or polyphthalamide is used as the plastic substrate.
It is important that the bonding material 1118 is a material having a higher adhesiveness between the stack body 1151 including the insulating film 1103 and the third substrate 1117 than the adhesiveness between the second substrate 1115 and the stack body 1151 that is a separation layer, of the adhesive material 1116 formed of an organic material.
As the bonding material 1118, various kinds of curable bonding materials such as a reactive curable bonding material, a heat-curable bonding material, a light-curable bonding material such as a UV-curable bonding material, and an anaerobic curable bonding material can be used.
Alternatively, an adhesive material may be provided on the insulating film 1103 instead of performing the above steps. In this case, a release paper (a release liner, that is, a separator or the like, in which a sheet having a separation surface on one or each substrate surface) may be provided so that the adhesive material does not adhere to other members. When a release liner is peeled, the adhesive material can adhere to any member; therefore, a substrate is not required and in addition, a thin semiconductor device can be provided.
Next, as shown in FIG. 2D, the adhesive material 1116 and the second substrate 1115 are separated from the stack body 1151. The adhesive material 1116 made of an organic material is subjected to thermal reaction, photoreaction, reaction to humidity, or chemical reaction (adhesion is decreased using for example, water, oxygen, or the like), and then the adhesive material 1116 and the second substrate 1115 are separated from the stack body 1151 formed from an organic material.
Through the above steps, as shown in FIG. 2E, a semiconductor device having a TFT formed from a polysilicon film and an element formed from an amorphous silicon film over a plastic substrate can be formed.